This project will exploit the power of cellular and genetic approaches in zebrafish to illuminate the functions of new genes that are essential for Schwann cell development and myelination in the vertebrate nervous system. In the peripheral nervous system, Schwann cells form the myelin sheath that wraps axons and allows for the rapid transmission of action potentials. Disruption of myelin causes peripheral neuropathies, debilitating diseases that affect 1.5 million people in the United States. Gaps in the understanding of the signals that govern the formation of myelin have hindered the development of therapies for the repair of myelinated axons. In genetic screens for mutations that disrupt myelination, we identified 13 mutations in 10 different genes with specific functions in the development of myelinated axons. Cellular and molecular studies demonstrate that the mutations define genes that function at many steps of the development of myelinated axons, including glial fate specification, Schwann cell migration, organization of the nodes of Ranvier, transport of specific myelin mRNAs within myelinating glia, and commitment of Schwann cells to myelination. In this application, we focus on a group of genes that have key functions in Schwann cell development and myelination. (1) In previous studies, we have identified mutations in erbb2 and erbb3, genes that encode components of a heteromeric receptor for Neuregulin (Nrg) signals, and showed that ErbB signaling is essential for directed migration of Schwann cells. In the present application, we propose to identify the operative signals and test the hypothesis that two phases of Nrg signaling direct Schwann cells as they migrate along axons of growing peripheral nerves. We propose to determine which Nrg isoforms guide Schwann cell migration, to determine whether these isoforms are expressed by and required in neurons or Schwann cells, and to determine whether Nrg signals are sufficient to guide migrating Schwann cells to ectopic locations. (2) In recent work, we found that two of our mutations disrupt a member of the G-protein coupled receptor superfamily. We propose to determine whether the protein acts as a signal or a receptor, or both, and to test the hypothesis that the gene acts in neurons to instruct Schwann cells to initiate myelination. (3) Our preliminary studies show that the st64 gene, which we are working to identify by positional cloning, is required for an early step in Schwann cell development. In analyses of marker gene expression, st64 mutants have a phenotype very similar to a partial loss of erbb3 function. Our preliminary studies suggest that analysis of the st64 mutation will define the function of a novel gene with an essential role in Schwann cell development. We propose to define the cellular and biochemical functions of the st64 gene by phenotypic analysis of the mutants and molecular analysis of the mutated gene.